Devices known as “white LED's” are relatively recent innovations designed to replace the conventional incandescent light bulb. It was not until LED's emitting in the blue/ultraviolet region of the electromagnetic spectrum were developed that it became possible to fabricate a white light illumination source based on an LED. Economically, white LED's have the potential to replace incandescent light sources (light bulbs), particularly as production costs fall and the technology develops further. In particular, the potential of a white light LED is believed to be superior to that of an incandescent bulb in lifetime, robustness, and efficiency. For example, white light illumination sources based on LED's are expected to meet industry standards for operation lifetimes of 100,000 hours, and efficiencies of 80 to 90 percent. High brightness LED's have already made a substantial impact on such areas of society as traffic light signals, replacing incandescent bulbs, and so it is not surprising that they will soon provide generalized lighting requirements in homes and businesses, as well as other everyday applications. The term “white LED” may be something of a misnomer as no LED emits “white light,” but it is used throughout the art to describe a lighting system where a blue/UV LED provides energy to another component of the system, one or more phosphors, which emit light when excited by the pumping LED, and where the excitation radiation from the pumping LED is combined with the light from the phosphor(s) to produce the final white light “product.”
As described in U.S. Pat. No. 7,476,338 to Sakane et al., there are in the art generally two approaches to providing LED-based white light illumination systems. In a conventional multi-chip type system the three primary colors are provided by red, green, and blue LEDs individually. A one-chip system comprises a blue LED in conjunction with a phosphor where the blue LED serves two purposes: the first being to excite the phosphor, and the second to contribute blue light which is combined with the light emitted by the phosphor to make the perceived white light combination.
According to Sakane et al. the one-chip type system has a preferable characteristic in that an LED-phosphor system can be dimensionally smaller than a multi-chip system, and simpler in design because the multiple drive voltages and temperature considerations of multiple LEDs do not have to be taken into account. Thus the cost to manufacture the system may be reduced. Further, by using a phosphor having a broad emission spectrum, the white emission from the system better approximates the spectrum of sunlight, and thus the color rendering properties of the system may be improved. For these reasons greater attention has been given to the one-chip rather than multiple-chip type systems.
The single-chip type systems may further be divided into two categories. In a first category, as alluded to earlier, light from a high luminescence blue LED and a phosphor emitting a yellow color as a result of excitation from the blue LED is combined, the white luminescence of the combined light obtained by using a complementary relation between the blue emission of the LED and the yellow emission of the phosphor. In the second category, the excitation source is an LED that emits in the near-ultraviolet or ultraviolet (UV) region of the spectrum, and the light from the phosphor package may include a blue-emitting phosphor, red-emitting phosphor, and green-emitting phosphor is combined to form white light. In addition to being able to adjust the color rendering properties of the white light with this type of system, an arbitrary emission color may also be produced by controlling the mixing ratios of the red, green, and blue photoluminescence.
The benefits of these single-chip systems are well appreciated in the art, but so too are their drawbacks when it comes to enhancing color rendering properties. For example, the white light emission from a typical one-chip system consisting of a blue LED and a yellow phosphor (such as YAG:Ce) is deficient in the longer wavelength side of the visible spectrum, resulting in a bluish white light appearance. The YAG:Ce yellow phosphor of the system does not help much in contributing to the needed 600 to 700 nm emission content, since its excitation band with the greatest efficiency is at about 460 nm, and the excitation range of this yellow phosphor is not particularly broad. Further disadvantages of this single-chip system are the disparities in the emission wavelength ranges of the blue LED, due in part to the manufacturing process, and if these deviate from the optimal excitation range of the YAG:Ce-based yellow phosphor, there results a loss of wavelength balance between the blue light and the yellow light.
There are also disadvantages to this second category of single-chip systems. White light illumination formed by combining the photoluminescence from a UV or near-UV excited red, green, and blue phosphor system is also deficient in the longer wavelengths because the excitation and emission efficiencies of the red phosphor are lower compared to that of the other phosphors in the package. The white LED designer therefore may have little choices available other than to increase the ratio of the red phosphor in the mixture relative to the blue and green phosphors. But this action may lead to an undesirable consequence: the ratio of the green phosphor to the others may now be insufficient and luminescence from the white LED may suffer. It would appear that a white color with high luminescence is difficult to obtain. And the color rendering properties are still nowhere near optimum as the red phosphor typically has a sharper emission spectrum than the others.
It is clear that multi-chip white light illumination systems suffer from disadvantages, not the least of which is a need for a plurality of voltage control systems and the increased heat production from the many individual chips needed to produce the white light's component colors. But each of the single-chip systems have their problems too, perhaps most notably being the inability to achieve an acceptable color rendering outcome. What is needed in the art is a white light illumination system with enhanced luminosity and color rendering, while at the same time achieving a balance with the need for more sophisticated drive and control systems.
Furthermore, there is a need for more power efficient illumination systems for Liquid Crystal Displays (LCD) that can meet the exacting industry specifications.